The present invention relates to an image heating device that is used in an image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus and includes a heat generating source for thermally fixing an unfixed image, which employs an electromagnetic induction heating method, and an image forming apparatus using the same.
Image heating devices employing electromagnetic induction are disclosed in JP2000-181258 A and JP2000-206813 A
FIG. 27 is a cross-sectional view of the image heating device disclosed in JP2000-181258 A. FIG. 28 is a front view showing a moving mechanism of a fixing device used in the image heating device. In FIG. 27, reference numerals 101 and 102 denote a heating roller that generates heat by induction heating and is rotated and a pressurizing roller that makes contact under pressure with the heating roller 101, respectively. A recording material (sheet) 105 is passed through a pressure-contacting portion between both the rollers 101 and 102, so that an unfixed image on the recording material 105 is fixed. Further, reference numerals 103 and 104 denote an excitation coil that is arranged on an outer periphery of the heating roller 101 and generates a high-frequency magnetic field, and a magnetic field shielding material that regulates an amount of heat to be generated, respectively.
The recording material 105 carrying the unfixed toner image is conveyed to a nip portion defined by the heating roller 101 and the pressurizing roller. Then, the toner image on the recording material 105 is fixed by heat of the heating roller 101 and pressure of the pressurizing roller 102.
The magnetic field shielding material 104 is, as shown in FIG. 28, divided into a plurality of portions in a width direction of the recording material 105. The magnetic field shielding materials 104 as the portions of the divided magnetic field shielding material 104 are housed in three separate cases, i.e. a case 104a arranged in a center portion so as to correspond to a passing area PA4L through which a JIS size A4 paper sheet is passed in a longitudinal direction, and cases 104b and 104c arranged on both outer sides of the case 104a. A distance between the respective outer side ends of the cases 104b and 104c corresponds to a passing area PA4T (PA4T greater than PA4L) through which a JIS size A4 paper sheet is passed in a lateral direction. The cases 104b and 104c on both outer sides can be raised or lowered by a case moving mechanism 108 that is composed of a shaft 106 with a thread groove formed on an outer periphery and a sliding portion 107 provided with an internal thread that is threaded in the thread groove. When passing A4-sized paper sheets continuously in the longitudinal direction, the cases 104b and 104c on both the outer sides are retracted upward so that the magnetic field shielding materials 104 housed therein are moved away from the excitation coil 103. Thus, in portions opposed to the cases 104b and 104c, a magnetic flux reaching the heating roller 101 is weakened, thereby allowing a temperature rise of the heating roller 101 in the portions to be suppressed. When passing an A4-sized paper sheet in the lateral direction, the cases 104b and 104c on both the outer sides are lowered. Thus, an amount of heat generated by the heating roller 101 can be made substantially uniform over the full width.
FIG. 29 shows a configuration of an induction heating circuit of an image heating device of an image forming apparatus disclosed in JP2000-206813 A. In the figure, three sets of induction heating portions, each composed of a magnetic core 201 and an induction heating coil 202, are arranged so as to be opposed to a fixing roller 203. The induction heating portion in the center is supplied with power from a center portion induction heating power supply 205, and the induction heating portions at both ends are supplied with power from an end portion induction heating power supply 207. In a center portion and an end portion, temperature detecting portions TH1 and TH2 are provided, respectively. According to a detected temperature, the power supply to each of the induction heating portions is controlled. In this configuration, when heat is radiated to a greater degree in both the end portions than in the center portion of the fixing roller 203, a larger amount of power is injected into the induction heating coils opposed to the end portions. When a larger amount of heat is lost in the center portion of the fixing roller 203 as in the case where a paper sheet of a small width is passed, a reduced amount of power is supplied to the induction heating coils opposed to the end portions. In this manner, a temperature of the fixing roller 203 in an axial direction is kept uniform.
However, the image heating device FIGS. 27 and 28) disclosed in JP2000-181258 A has presented the following problems.
First of all, in this configuration, a core of a magnetic material is not present in an inner peripheral portion of the excitation coil 103, and thus magnetic coupling between the excitation coil 103 and the heating roller 101 does not work well. Therefore, in order for the heating roller 101 to be heated to a desired temperature by induction heating, a large electric current is required, thereby making an excitation circuit costly. Furthermore, because of a configuration in which the magnetic field shielding materials 104 are moved according to a width of a paper sheet to be passed, passing various types of paper sheets results in many combinations of the magnetic field shielding material to be moved and the magnetic field shielding material not to be moved. This requires a plurality of moving mechanisms, thereby making the configuration complicated and costly. Moreover, a space for moving the magnetic field shielding materials 104 and a space for the moving mechanism are required. Thus, the fixing device is made bulky, thereby making a whole image forming apparatus bulky, which has been disadvantageous.
The image heating device (FIG. 29) disclosed in JP2000-206813 A has presented the following problems.
First of all, a plurality of the induction heating portions, each composed of the magnetic core 201 and the induction heating coil 202, and a plurality of the induction heating power supplies are required, thereby making the device costly. Further, because of a configuration in which the induction heating portions and the induction heating power supplies are provided according to the sizes of paper sheets to be passed, when passing various types of paper sheets, a cost increase becomes considerable. For example, in order to achieve the passing of paper sheets varying in size between a maximum of JIS size A3 and a minimum of a post card size, and further to achieve the feeding of A4-sized and B5-sized paper sheets in longitudinal and lateral directions, it is necessary to provide five to seven induction heating portions, thereby making the device more costly. Furthermore, spaces for housing the plurality of the induction heating power supplies are required. Thus, the device is increased in size, which has been disadvantageous.
In order to solve these problems of the conventional image heating devices, it is an object of the present invention to provide an image heating device that can heat a heat generating roller uniformly in a width direction of a paper sheet to be passed. Further, it is another object of the present invention to provide an image heating device that is reduced in size and weight, in which an amount of heat generated by a heat generating roller can be controlled easily at low cost according to a width of a paper sheet to be passed. Moreover, it is still another object of the present invention to provide an image forming apparatus that includes the image heating device as a thermal fixing device.
In order to achieve the aforementioned objects, the present invention has the following configurations.
An image heating device of a first configuration according to the present invention includes a heat generating member of a conductive material, an excitation unit that is arranged in the vicinity of the heat generating member and generates an annular magnetic flux to cause the heat generating member to generate heat by electromagnetic induction, and a heat generation suppressing unit that suppresses heat generation of the heat generating member by suppressing the magnetic flux generated by the excitation unit.
According to this configuration, a distribution of an amount of heat generated in a width direction can be regulated arbitrarily so as to correspond to a width of a paper sheet and a temperature of the heat generating member. Thus, the heat generating member can be heated uniformly in the width direction of the paper sheet.
In the above image heating device of the first configuration, preferably, the heat generation suppressing unit includes a conductor arranged in a path of the annular magnetic flux generated by the excitation unit, and the conductor induces a loop-shaped electric current linking to the magnetic flux under the magnetic flux. This configuration allows the heat generation suppressing unit to be constructed easily at low cost.
Preferably, with respect to a common annular magnetic flux generated by the excitation unit, a plurality of the conductors are provided. According to this configuration, an action of the heat generation suppressing unit can be regulated more freely, thereby allowing temperature regulation of the heat generating member to be performed precisely.
Preferably, the excitation unit includes an excitation coil arranged so as to be opposed to the heat generating member and a core of a magnetic material. This configuration allows the heat generating member to generate heat efficiently.
Preferably, the heat generation suppressing unit includes an additional coil wound around the core. According to this configuration, magnetic coupling between the excitation unit and the heat generation suppressing unit can be enhanced, thereby allowing the action of the heat generation suppressing unit to be enhanced. Further, the heat generation suppressing unit can be constructed easily at low cost and reduced in size. Moreover, changing a wire constituting the coil and how the wire is wound makes it easy to change a heat generation suppressing effect desirably.
An image heating device of a second configuration according to the present invention includes a heat generating member of a conductive material, an excitation unit, and a heat generation suppressing unit. The heat generating member has a rotatable cylindrical face. The excitation unit includes an excitation coil arranged so as to be opposed to the heat generating member and a core of a magnetic material and generates an annular magnetic flux to cause the heat generating member to generate heat by electromagnetic induction. The heat generation suppressing unit suppresses heat generation of the heat generating member by suppressing a magnetic flux generated by the excitation unit. The excitation coil is formed of a wire wound in the following manner. In end portions of the cylindrical face of the heat generating member in a rotation axis direction, the wire is wound along outer peripheral faces of the end portions. In portions other than the end portions, the wire is wound along a generatrix direction of the cylindrical face. The core is arranged so as to cover the excitation coil in a rotation direction of the cylindrical face, on an opposite side of the heat generating member with respect to the excitation coil. The core includes a magnetically permeable portion opposed to the heat generating member through the excitation coil and an opposing portion opposed to the heat generating member without interposing the excitation coil between them. The heat generation suppressing unit includes an additional coil wound around the core.
According to this configuration, the annular magnetic flux passing through the core, which is generated by the excitation coil, is suppressed, so that a temperature of the heat generating member in the rotation axis direction is made uniform. Further, by changing a specification of the additional coil, the degree to which a magnetic flux generated by the excitation unit is suppressed easily can be set arbitrarily.
In the above image heating device, preferably, both ends of the additional coil are short-circuited. According to this configuration, a change in the annular magnetic flux generated by the excitation unit causes an induction current to be generated in the additional coil, so that a magnetic flux that suppresses the annular magnetic flux is generated. As a result, the heat generation in a portion of the heat generating member can be suppressed, which corresponds to a portion in which the additional coil is provided.
Furthermore, in the above image heating device, preferably, the heat generation suppressing unit further includes a switching unit connected in series to the additional coil. According to this configuration, an amount of heat generated by the heat generating member in the rotation axis direction can be regulated at any time according to a paper width and a temperature of the heat generating member.
Preferably, the additional coil is wound around the magnetically permeable portion. According to this configuration, magnetic coupling between the excitation unit and the heat generation suppressing unit is enhanced, thereby allowing the action of the heat generation suppressing unit to be enhanced. Further, the heat generation suppressing unit can be constructed easily at low cost and reduced in size. Moreover, changing the wire constituting the coil and how the wire is wound makes it easy to change the heat generation suppressing effect desirably.
Preferably, the core includes a plurality of the magnetically permeable portions, and the additional coil is wound around at least one of the plurality of the magnetically permeable portions. This configuration allows a temperature of the heat generating member to be made uniform over the full width.
Preferably, a plurality of the additional coils are wound around the common magnetically permeable portion of the core. This configuration allows temperature regulation to be performed more freely and precisely.
Preferably, a pair of the additional coils are wound around the core, and the pair of the additional coils are wound in opposite directions. According to this configuration, the additional coils provided on both sides of the core suppress magnetic flux, respectively, and thus the heat generation suppressing effect is enhanced compared with the case of suppressing heat generation using the additional coil provided only on one side.
Preferably, the pair of the additional coils are wound around the core, and the pair of the additional coils and the switching unit are connected in series. According to this configuration, an action of the pair of the additional coils provided on the core can be switched over using one connecting/disconnecting unit.
Preferably, the additional coil is formed of a wire bundle of wires with insulated surfaces. According to this configuration, electric resistance with respect to a high-frequency alternating current induced in the additional coil is decreased, thereby allowing a larger electric current to be obtained using an additional coil of the same number of turns. Thus, a magnetic flux suppressing effect further can be enhanced.
Preferably, the excitation coil is formed of a wire bundle of the wires with their surfaces insulated. According to this configuration, electric resistance of the excitation coil is decreased, thereby allowing the supplied power to be converted into heat generation of the heat generating member efficiently.
Preferably, with respect to a common annular magnetic flux generated by the excitation unit, a plurality of the additional coils are provided. According to this configuration, the action of the heat generation suppressing unit can be regulated more freely, thereby allowing temperature regulation of the heat generating member to be performed precisely.
Preferably, the additional coil is arranged on an outer side of a minimum-sized paper passing area. According to this configuration, when small-sized paper sheets are passed continuously, an excessive temperature rise of the heat generating member in an area other than a passing area of the paper sheets can be prevented.
Preferably, a plurality of the additional coils are arranged on the outer side of the minimum-sized paper passing area, and the switching unit is switched over according to a width of a paper sheet to be passed. This configuration allows the heat generation suppressing unit to function so as to correspond to a width of a paper sheet to be passed. Thus, even when paper sheets varying in size are passed, a temperature of the heat generating member in the rotation axis direction always can be kept uniform.
Moreover, preferably, a temperature detecting device is provided, and the switching unit is switched over according to a temperature detected by the temperature detecting device. According to this configuration, a temperature of the heat generating member in the rotation axis direction always can be maintained uniformly without detecting a width of a paper sheet to be passed.
Preferably, when no paper is passed, the switching unit is brought to an unconnected state, and after the passing of paper is started, the switching unit is switched to a connected state. According to this configuration, after the heat generating member is heated uniformly in the rotation axis direction, the switching unit is switched over according to a paper width or a temperature, so that an excessive temperature rise in end portions of the heat generating member can be prevented, and fixing variations also can be prevented.
Preferably, at temperatures lower than a set temperature, the switching unit is brought to the unconnected state, and after the set temperature is attained, the switching unit is switched to the connected state. According to this configuration, after the heat generating member is heated uniformly in the rotation axis direction, the switching unit is switched over according to a paper width or a temperature, so that an excessive temperature rise in the end portions of the heat generating member can be prevented, and fixing variations also can be prevented.
Preferably, at temperatures lower than the set temperature, the switching unit is switched over according to a width of a paper sheet to be passed. According to this configuration, only a portion corresponding to the width of the paper sheet is heated, thereby allowing a reduction in power consumption and a shortening of temperature raising time to be achieved.
In the above image heating device of the second configuration, preferably, the core includes a plurality of substantially U-shaped cores, and the plurality of the U-shaped cores are arranged so as to cover the cylindrical face of the heat generating member in the rotation direction, at a distance from each other in the rotation axis direction of the heat generating member. According to this configuration, the excitation coil can radiate heat from gaps between the cores, and at the same time, surface areas of the cores themselves are increased, and thus heat radiation from the cores can be enhanced, thereby allowing a temperature rise of the cores and the coil to be prevented.
Preferably, the core further includes a second core portion that magnetically connects the plurality of the U-shaped cores, and the second core portion includes an opposing portion opposed to the heat generating member without interposing the excitation coil between them. According to this configuration, a magnetic flux generated by the excitation unit can be dispersed in the rotation axis direction of the heat generating member, thereby allowing an amount of heat generated by the heat generating member in the rotation axis direction to be made uniform.
Preferably, only a portion of the plurality of the U-shaped cores is provided with the additional coil. This configuration allows a temperature of the heat generating member to be made uniform in the rotation axis direction.
Preferably, substantially a center portion of the U-shaped core is connected to the second core portion. According to this configuration, in each U-shaped core, two annular magnetic fluxes can be generated, thereby allowing the heat generating member to generate heat efficiently.
Preferably, the U-shaped core is arranged so as to be inclined with respect to the rotation axis direction of the heat generating member. According to this configuration, the positions of the opposing portions in the rotation axis direction of the heat generating member can be dispersed, and the opposing portions can be arranged at a smaller distance from each other in the direction. Thus, temperature variations in the rotation axis direction of the heat generating member can be reduced.
Alternatively, the above image heating device of the second configuration may have the following configuration. That is, the core includes a plurality of substantially L-shaped cores, and the plurality of the L-shaped cores are arranged so as to cover the cylindrical face of the heat generating member in the rotation direction, at a distance from each other in the rotation axis direction of the heat generating member. According to this configuration, the excitation coil can radiate heat from gaps between the cores, and at the same time, surface areas of the cores themselves are increased, and thus heat radiation from the cores can be enhanced, thereby allowing a temperature rise of the cores and the coil to be prevented. Further, the amount of a material of the core is reduced, and thus the device can be reduced in size and weight and manufactured at lower cost. Furthermore, since a heat radiation property is improved, the L-shaped cores can be arranged at a smaller distance from each other in the rotation axis direction of the heat generating member. As a result, temperature variations in the rotation axis direction can be reduced.
Preferably, the core further includes a second core portion that magnetically connects the plurality of the L-shaped cores, and the second core portion includes an opposing portion opposed to the heat generating member without interposing the excitation coil between them. According to this configuration, a magnetic flux generated by the excitation unit can be dispersed in the rotation axis direction of the heat generating member, thereby allowing an amount of heat generated by the heat generating member in the rotation axis direction to be made uniform.
Preferably, only a portion of the plurality of the L-shaped cores is provided with the additional coil. This configuration allows a temperature of the heat generating member to be made uniform in the rotation axis direction.
Preferably, one end portion of the L-shaped core is connected to the second core portion. This configuration allows one annular magnetic flux to be generated in each of the L-shaped cores. Thus, in the heat generating member, a difference between the amounts of heat generated in a portion opposed to the L-shaped core and a portion other than the portion opposed to the L-shaped core can be decreased, thereby allowing temperature variations in the rotation axis direction to be reduced.
Preferably, the L-shaped cores are provided in a staggered arrangement with respect to the second core portion. According to this configuration, since the heat radiation property is improved, the L-shaped cores can be arranged at a smaller distance from each other in the rotation axis direction of the heat generating member. As a result, temperature variations in the rotation axis direction can be reduced.
Preferably, the opposing portion of the core includes a convex portion protruding to a side of the heat generating member. According to this configuration, magnetic coupling between the excitation unit and the heat generating member is enhanced, thereby allowing the heat generating member to generate heat efficiently.
Preferably, the opposing portion of the second core portion includes a convex portion protruding to a side of the heat generating member, and the convex portion is inserted in a hollow portion in a winding center of the excitation coil. According to this configuration, magnetic coupling between the excitation unit and the heat generating member is enhanced, thereby allowing the heat generating member to generate heat efficiently.
An image heating device of a third configuration according to the present invention includes a heat generating member of a conductive material, an excitation power supply that generates an electric current changing over time, an excitation unit that is arranged in the vicinity of the heat generating member and supplied with the electric current from the excitation power supply to generate an annular magnetic flux so as to cause the heat generating member to generate heat by electromagnetic induction, and a heat generation suppressing unit including a conductor that is arranged in a path of the annular magnetic flux generated by the excitation unit and induces a loop-shaped electric current linking to the magnetic flux under the magnetic flux, and a switching unit for passing and interrupting the electric current. The switching unit is switched over when an induction current generated in the conductor has a value close to zero.
According to this configuration, at the moment when an electric current of the same waveform as that of a high-frequency current fed to the excitation unit, which is induced in the conductor under the high-frequency current, has a value of substantially zero, the switching unit can be switched over. Thus, the generation of an excessively high voltage in the switching unit and the occurrence of sparking and insulation destruction can be prevented. At the same time, abrupt changes in electric current and voltage are prevented from being caused in the conductor due to switching of the switching unit, thereby allowing the generation of unwanted electromagnetic noise to be prevented.
An image heating device of a fourth configuration according to the present invention includes a heat generating member of a conductive material, an excitation power supply that generates an electric current changing over time, an excitation unit that is arranged in the vicinity of the heat generating member and supplied with the electric current from the excitation power supply to generate an annular magnetic flux so as to cause the heat generating member to generate heat by electromagnetic induction, and a heat generation suppressing unit including a conductor that is arranged in a path of the annular magnetic flux generated by the excitation unit and induces a loop-shaped electric current linking to the magnetic flux under the magnetic flux, and a switching unit for passing and interrupting the electric current. The switching unit is switched over when an induction voltage generated in the conductor has a value close to zero.
According to this configuration, at the moment when a voltage of the same waveform as that of a high-frequency current fed to the excitation unit, which is induced in the conductor under the high-frequency current, has a value of substantially zero, the switching unit can be switched over. Thus, the generation of an excessively high voltage in the switching unit and the occurrence of sparking and insulation destruction can be prevented. At the same time, abrupt changes in electric current and voltage are prevented from being caused in the conductor due to switching of the switching unit, thereby allowing the generation of unwanted electromagnetic noise to be prevented.
In the above configuration, preferably, when switching over the switching unit, no electric current is applied to the excitation unit. According to this configuration, the switching unit can be switched over in a state where an electric current or a voltage of the same waveform as that of a high-frequency current fed to the excitation unit, which is induced in the conductor under the high-frequency current, has a value of zero. Thus, the generation of an excessively high voltage in the switching unit and the occurrence of sparking and insulation destruction can be prevented. At the same time, abrupt changes in electric current and voltage are prevented from being caused in the conductor due to switching of the switching unit, thereby allowing unwanted electromagnetic noise to be prevented.
An image heating device of a fifth configuration according to the present invention includes a heat generating member of a conductive material, an excitation power supply that generates an electric current and a voltage that change over time, an excitation unit that is arranged in the vicinity of the heat generating member and supplied with the electric current and the voltage from the excitation power supply to generate an annular magnetic flux so as to cause the heat generating member to generate heat by electromagnetic induction, and a heat generation suppressing unit including a conductor that is arranged in a path of the annular magnetic flux generated by the excitation unit and induces a loop-shaped electric current linking to the magnetic flux under the magnetic flux, and a switching unit for passing and interrupting the electric current. The switching unit is switched over in synchronization with changes in the electric current or the voltage supplied to the excitation unit.
According to this configuration, at the moment when an electric current or a voltage of the same waveform as that of a high-frequency current fed to the excitation unit, which is induced in the conductor under the high-frequency current, has a value of substantially zero, the switching unit can be switched over. Thus, the generation of an excessively high voltage in the switching unit portion and the occurrence of sparking and insulation destruction can be prevented. At the same time, abrupt changes in electric current and voltage are prevented from being caused in the conductor due to switching of the switching unit, thereby allowing the generation of unwanted electromagnetic noise to be prevented.
An image heating device of a sixth configuration according to the present invention includes a heat generating member of a conductive material, an excitation power supply that generates an electric current changing over time, an excitation unit that is arranged in the vicinity of the heat generating member and supplied with the electric current from the excitation power supply to generate an annular magnetic flux so as to cause the heat generating member to generate heat by electromagnetic induction, and a heat generation suppressing unit including a conductor that is arranged in a path of the annular magnetic flux generated by the excitation unit and induces a loop-shaped electric current linking to the magnetic flux under the magnetic flux, and a switching unit for passing and interrupting the electric current. The conductor is formed of a wire wound with at least one turn.
According to this configuration, a magnetic flux suppressing action is enhanced, thereby allowing the effect of controlling a temperature distribution to be enhanced. When the conductor of an increased number of turns is used, a suppressing action upon a magnetic flux generated by the excitation unit further is enhanced. Further, by changing the number of turns according to temperature ununiformity, temperature uniformity of the heat generating member in the rotation axis direction can be regulated.
In the above configuration, preferably, the wire is wound with at least two turns whose paths are different from each other in at least a portion. According to this configuration, magnetic fluxes in a plurality of positions can be controlled using the single switching unit. Thus, a controlling operation can be performed more precisely using a reduced number of the switching units, and a uniform temperature distribution can be realized.
Preferably, the respective turns of the wire are wound apart from each other. According to this configuration, an area in which the conductor is provided can be increased using a reduced amount of the wire, thereby allowing a heat generation suppressing effect of this conductor to be enhanced.
An image heating device of a seventh configuration according to the present invention includes a heat generating member of a conductive material, an excitation power supply that generates an electric current changing over time, an excitation unit that is arranged in the vicinity of the heat generating member and supplied with the electric current from the excitation power supply to generate an annular magnetic flux so as to cause the heat generating member to generate heat by electromagnetic induction, and a heat generation suppressing unit including a conductor that is arranged in a path of the annular magnetic flux generated by the excitation unit and induces a loop-shaped electric current linking to the magnetic flux under the magnetic flux, and a switching unit for passing and interrupting the electric current. The conductor has a length in a direction along the annular magnetic flux that is greater than a thickness of the conductor in a plane perpendicular to the direction along the annular magnetic flux.
According to this configuration, while a heat generation suppressing action of the conductor is secured sufficiently, the conductor can be reduced in size and formed from a reduced amount of a material.
Preferably, the heat generation suppressing unit suppresses the magnetic flux generated by the excitation unit by generating a magnetic flux in an opposite direction to a direction of the magnetic flux generated by the excitation unit.
More specifically, preferably, the heat generation suppressing unit generates an induced electromotive force under the magnetic flux generated by the excitation unit to induce an electric current, so that a magnetic flux in a direction in which the magnetic flux generated by the excitation unit is cancelled out is generated.
According to this configuration, heat generation of the heat generating member can be suppressed by a simple method, and according to a paper width and a temperature distribution of the heat generating member in the rotation axis direction, an amount of heat generated by the heat generating member in the rotation axis direction can be controlled arbitrarily.
Preferably, the conductor includes a hollow portion through which the magnetic flux is passed. According to this configuration, using the heat generation suppressing unit that is reduced in size by reducing an amount of a material of the conductors, the capability of regulating a heat generation distribution can be secured.
Preferably, the conductor is formed of a wound wire. This configuration allows the heat generation suppressing unit to be constructed easily at low cost. Further, changing the wire and how the wire is wound makes it easy to change the heat generation suppressing effect desirably.
Alternatively, the conductor may be formed of a wound belt-like material. This configuration makes it easier to construct and mount the heat generation suppressing unit.
Preferably, the conductor has an electric conductivity of not less than 1xc3x97107 [S/m]. According to this configuration, the conductor can be prevented from generating heat under an electric current induced in the conductor. Further, an electric current value of the induced electric current becomes high, thereby allowing the heat generation suppressing effect to be enhanced.
Preferably, a magnetic material is provided on an inner side or in the vicinity of the conductor. According to this configuration, magnetic coupling between the excitation unit and the conductor is enhanced, and thus the heat generation suppressing effect provided by an electric current induced in the conductor can be enhanced.
Preferably, a distance between an end portion of the magnetic material and the conductor along the annular magnetic flux is greater than a length of the conductor along the annular magnetic flux. This configuration allows the heat generation suppressing action of the conductor to be enhanced.
Preferably, the conductor is inclined with respect to the annular magnetic flux penetrating the conductor. According to this configuration, the heat generation suppressing action of the conductor in a direction orthogonal to the annular magnetic flux can be changed continuously. Thus, an amount of heat to be generated can be controlled more precisely, thereby allowing a desired temperature distribution to be attained.
The image heating device of the present invention further may include a thin fixing belt and a fixing roller for suspending the fixing belt so that the fixing belt is suspended between the fixing roller and the heat generating member. According to this configuration, the respective materials, thicknesses, or the like of the heat generating member and the fixing belt can be set independently, thereby allowing optimum materials and thicknesses for heating, raising temperature, fixing, or the like to be set.
An image forming apparatus according to the present invention includes an image forming unit in which an unfixed image is formed on a recording material and carried by the recording material and a thermal fixing device that thermally fixes the unfixed image on the recording material. The thermal fixing device is formed of the image heating device of the present invention. Thus, an image forming apparatus can be provided that is reduced in size and weight and allows cost reduction, in which recording materials varying widely in size can be processed using a simple configuration.